A piston for internal-combustion engine is generally made of aluminum alloy, and its peripheral portion is formed with a plurality of piston ring grooves. A piston ring is fitted in this piston ring groove. Of the piston ring grooves, the top ring groove, in which the top ring positioned nearest to the crown surface is fitted, tends to have abrasion on its inner surface, because of the reasons that the temperature is high due to its close position to the combustion chamber and that the top ring receives the combustion pressure, and because of other reasons.
As a countermeasure against this, Japanese Patent Laid-open Publication 2-125952 proposes an aluminum alloy piston prepared by forming a peripheral groove on a peripheral portion forming thereon a piston ring groove, then by fitting into the peripheral groove a copper wire material wound thereon, and then by locally melting the copper wire material and its neighboring piston base material by electron beam, then by allowing the melted copper to diffuse into the piston base material, thereby to form an abrasion-resistant hardened layer made of an alloy of aluminum alloy and copper, having a copper content of 10-35 wt %, and then by forming the piston ring groove on the hardened layer.
According to the above-mentioned conventional example, however, the area of the primary crystal layer is large in a hardened layer's region which has a small copper content. Therefore, it tends to adhere to a cast-iron piston ring during the driving of the internal-combustion engine. Furthermore, the amount of an intermetallic compound of aluminum alloy and copper becomes large in a region having a large copper content. Thus, the hardened layer may have cracks due to thermal stress after the electron beam irradiation.
As a countermeasure against this, it is supposed to melt only a bonded portion between a piston base material and a composite material formed from a copper-containing powder compact, in place of the copper wire material, and to form a piston ring groove on the composite material as the hardened layer superior in abrasion resistance. In this case, however, a low-melting-point binder powder of the powder compact vaporizes by the electron beam irradiation. Therefore, it is difficult to conduct a melt adhesion between the piston base material and the composite material, and thus it can not be adopted offhand.
The present invention was made in view of the above-mentioned problems of conventional technologies. Its object is to provide a piston for internal-combustion engine, having a piston ring groove superior in abrasion resistance, and a method for producing the same, without having adhesion of a hardened layer portion, which forms thereon a piston ring, to the piston ring, and without having cracks on the hardened layer.